1. Field of the Invention
The present invention relates to an outboard motor having a power transmission mechanism to transfer an output power from an engine to a propeller, and more particularly to an outboard motor having a speed change gear structure contained in the transmission mechanism to transform the output power of the engine supplied to the propeller.
2. Background Art
One conventional type of such a mechanism for an outboard motor is described in JP-B-2686517, for instance. JP-B-2686517 discloses an outboard motor in which a first drive shaft is connected to the output shaft of the engine, the second drive shaft is disposed coaxially with the first drive shaft by way of a transmission unit, and a reduction and reversing gear mechanism is arranged to transfer the thrust received by the second drive shaft to a propeller shaft.
The transmission unit is composed of first and second planetary gear trains, with the first and the second planetary gear trains being composed of a sun gear, multiple planetary gears, and an internal gear to which respective planetary gears are engaged.
The rotational center axis of the respective sun gears in the first and the second planetary gear trains are connected by a spindle so that the first and the second planetary gear trains are integrated. The spindle also has a brake mechanism that can be operated from the outside. The first drive shaft transfers its thrust to the internal gear of the first planetary gear train, while the rotational movement of the planetary gear arm for the first planetary gear train generated by the aforementioned thrust transfer is delivered to the second drive shaft. The planetary gear arm for the first planetary gear train is integrated with the internal gear of the second planetary gear train, and a one-way clutch is installed to allow the planetary gear arm for the second planetary gear train to rotate only in the given direction.
It is disclosed that once the brake mechanism is activated, the reduction ratio of the thrust transferred to the second drive shaft becomes smaller compared with such a reduction ratio prior to the activation of the brake mechanism.
In addition, some outboard motors installed on a boat have a first propeller and a second propeller disposed linearly fore and aft facing each other, with the first and the second propellers being rotated in opposite directions by means of a contra-rotating propeller mechanism, as disclosed in JP-A-Hei7-144695 and JP-A-Hei9-263294, for instance.
The exhaust gas is basically discharged into the water in JP-A-Hei7-144695 and JP-A-Hei9-263294. In the course of acceleration, however, the exhaust gas is discharged from the inside of the propeller boss according to JP-A-Hei7-144695, while the exhaust gas is discharged from the outside of the propeller boss according to JP-A-Hei9-263294.
In such conventional mechanisms, however, the transmission unit has the first and the second planetary gear trains arranged in a vertical 2-stage configuration. Thus, it is a complicated structure and a large space is required for its installation.
Generally, it is advantageous in terms of propeller efficiency to drive a propeller having a relatively large diameter and large pitch at low speed, resulting in a higher top speed and improved fuel economy. But, a large pitch propeller needs a larger driving torque during acceleration. Solutions such as providing a turbocharger that adds torque in the low speed range is not very useful when a large pitch propeller is used for the improvement of cruising performance.